Alexrd: Not only those with physical releases become hard or impossible to find, but the ones that had a digital-only release simply disappear forever.
As has pretty much happened with the recent Deadpool game.
Yeah, I don't understand why this is so difficult for games. I can go into a store and buy a DVD of the terrible XIII mini-series, but the game XIII had to be purged from sale everywhere. Hell, even the cheesy 90's Captain America movie made its way onto DVD and Blu-ray, but there's no chance we'll ever see some kind of re-release of the "Captain America and The Avengers"-beat 'em up from 1991.
If I were to guess, I'd say that it probably has to do with the general attitude of the video game industry. Before services like GOG came into existence, major publishers like EA had absolutely no interest in making their old games available again, because they refused to believe that there was any worth or profit in that. The majority of these publishers seem to have absolutely no regard for the preservation or possible longevity of their own works. They just throw releases onto the market as momentary distractions for the masses and move on.
You can also see this in their attitude towards backwards-compatibility, in which
Yahtzee Croshaw sees the creation of
"an industry without long-term memory" that is doomed to repeat the same idiotic mistakes over and over again.